In general, a large-sized transporter vehicle called a dump truck is provided with a liftable vessel (loading platform) on a frame of a vehicle body. The dump truck travels in a state where objects to be transported, which are composed of crushed stones or sand and earth, are loaded in a large quantity on the vessel (Patent Document 1).
A transporter vehicle of this type according to the conventional art comprises an automotive vehicle body, a loading platform that is provided on the vehicle body in such a manner as to be tiltable (liftable) thereon and on which objects to be transported are loaded, and a cab that is positioned in front of the loading platform and is provided in the vehicle body to define an operator's room therein. A steering handle is provided in the cab of the transporter vehicle, which an operator grips to steer a travelling direction of the vehicle.
The transporter vehicle according to the conventional art is equipped with a steering device called a power steering apparatus configured to perform a steering operation of a vehicle using a hydraulic force for being able to assist in a handle operation of an operator. This steering device comprises a steering handle, a steering valve for controlling supply and discharge of pressurized oil in response to an operation of the steering handle, and a steering cylinder for steering the vehicle by the pressurized oil supplied/discharged through the steering valve (Patent Document 2).
Recently, information concerning a steering angle of a steering wheel is required for adopting a posture control system of a vehicle, a parking assistance system for automatically steering a steering handle, and the like. There is known the configuration of detecting an operation angle of a power steering to meet this requirement (Patent Document 3). On the other hand, there is also known the configuration of detecting a position of the steering by a plurality of detectors (Patent Document 4).